You can use filters that aren't made for PSP with a free program called "Plug-In Commander Light". It's a real easy program to use once you do it a couple of times. Don't be scared by my seemingly long instructions. Most of it is the set up for the program, which will be done just once. The part highlighted in red at the bottom is the actual instructions on how to use the program. But you really need to read all of these instructions first.

Here's where to get this program... CLICK HERE

If the link is dead, just go to google.com and do a search for Plug-In Commander Light.

Before anything is said about the plug in commander light program,  it should be known that tubes, plugins, etc. should be installed somewhere other than in your PSP program files.  That's because PSP will load real slow if too many of these things are installed in it's program files.  I have my psp extras folder in my C drive, but it can be in your Documents folder or wherever.  I have each extra in it's own sub folder.  For example, I have the main PSP Extras folder in my C drive.  Then in that are sub folders...Plugins, Tubes, Masks.  In each of those are more sub folders.  For example, the Plugins folder has sub folders named for all of the plugins I have.  "Factory Filter Gallery A", "Xenofex", etc.  Before I download a plugin, I create a new folder named after the plugin in the plugin folder.  Then I download that plugin to it's own sub folder and unzip and install to that same folder.  It's important to keep them all in their own sub folder because otherwise it will be a mess because one plugin contains several files. 

2) You have to make PSP recognize the folder.  Do so by doing this...
3) In PSP, click on "File / Preferences / File Locations".
4) Click on the "Plug-in filters" tab.
5) Folder #1 is probably already filled in because it's the default folder.  So fill in Folder # 2 by clicking on the Browse button.
6) Find the path to where you created your new filter folder.
7) Highlight it and then click OK at the box.
8) Have the first 2 things here checked..."Enable filters...". and "Include sub folders when searching for filters".
9) Now OK the remaining box and all filters you put in the filter folder and it's sub folders should now work with PSP. 

You may have to restart PSP for the filters to take effect.
Open a graphic in PSP and find the filter effects by clicking on "Effects / Plug-in Filters".
 

Now to get to the plug in commander light program...

Open the program...it's named PiCoLight.exe
Just OK the "tip of the day" box.
In the first box, make sure the C drive is in the dropdown box.
Click on "File / Preferences"

At the General tab, I have nothing checked except the bottom set..."Associate file types with PiCo".
I have all of those checked under that category.

Under the FF/TF tab, I have Filter Factory 3.0 checked.
Under the Interface tab, I have Full Row Selection, and search for plugins in all sub folders checked. 
And under Choose FF Editor Interface, I have "Gearwheel Hubbub..." there.

Under the Folders tab, here is where you let it know where your plugins are saved to.
Just click on the ADD button and navigate to your folder and double click on it.
Put all folders that your plugins are at here.  You may have more than one like me.
You don't have to put your sub folders there, because you enabled that under the interface tab.

Under the Images tab, I have nothing there, but auto add is checked.
Under the Apps, I have nothing there either, but the auto add is checked there too.
Under the Plugin Types tab, I have everything checked.
Now OK this box.

Back at the program...

In the first box, click on the "File View" tab.
You should see your folders where your plugins are at here.
Double click on it and then find your sub folder where the plugin you want to convert is.  Keep clicking until you find the actual filter, then double click on that.

The Output Format Box will pop up.
Have Photoshop FF plugin there, and have filter name as file name checked.
Then press the convert button.
Then make sure the path is correct as to where the plug in is going to be installed, which should be the same place you downloaded it to.  Click on the Select Folder button to put it there.  Mine's always the right place by default.
Then click OK.

Then that's it!  I usually have to restart PSP to see the filters in the filter list.