FXP and MP3 are generically called audio codecs. Codec stands for compression decompression and is generally referring to video files w/mp3 being the compression method for the audio portion. There are many methods to achieve compression. Back when MP3 was first coming out I remember an article on another competing system that was suppose to result in smaller files with better quality sound that has gone the way of Betamax(if your old enough to remember betamax). I believed it was made by Yamaha. Foxpro paid to develop their very own audio codec so that users of these sounds could only use them on their machines, and to help protect them from copyright infringment.
Which is exactly what you are asking someone to do. In order for you to legally have those sounds, you must purchase them from foxpro, an authorized foxpro retailer, or PURCHASE THE ORGINAL SOUNDS FROM AN INDIVIDUAL WHO PURCHASED THE SOUNDS LEGALLY. In order for someone to sell you their foxpro sounds legally they would need to sell you their legally purchased copy and all back up copies they have made. This it like it is legal to buy a book from a retailer or even an origianl book from a book store or individual. However, you cannot legally buy a photocopy of a book. I doubt that you would really get in trouble for doing this, but whoever sells you the copies could really get into trouble if it became a big enough of a problem foxpro decided to push for a legal remedy.
As far as answering a coversion question, there are programs that will convert from WMA, WAV, MP3 bit rate x, MP3 bit rate y, VBR into various different formats. With Foxpro being such a small speciallized section of the audio market place, I would bet my next paycheck no one has written such a utility. Can it be done without programming a utility: yes. You would have to purchase the desired FXP sounds and a foxpro FXP compatable unit to play the sounds. You could probably play the sounds directly into you line in port on your computers sound card. BE VERY CAREFULL WITH THE VOLUME AND START AT THE LOWEST SETTING. If this blows your sound card out don't hold me responsible. Better way: Have someone with a good deal of electonics background take the foxpro apart and tap into the preamp right before the main amp if possible. This you could safely play into your sound card. The redneck way would require that you play your foxpro into a microphone running into your sound card. Now, you would need to capture/play your desired sounds into a wav format onto your computer thru your sound card. The next step would require that you get a program that could convert the wav to mp3. There are many such programs available. Some are even free or have a free 10 day try it before you buy trial period. Now you have finally accomplished what you are asking someone else to do for you. You could then play your sounds legally as long as you own the fxp sounds. If you deside to sell them you would be legally obligated to delete your mp3 sounds.
Final disclaimer, this is my interpretation of copyright laws. I could easily be wrong so I suggest you talk to a lawyer.
PS. I believe it would just be easier to either buy a foxpro or buy johnny stewart CDS. JS CDs already have MP3 formats already on them and they sound great.