I need to add Guitarron and Vihuela parts to my score.

I myself am not familiar with these instruments, but judging by their Wikipedia articles, it seems you could get a similar result by editing the string data for the Baritone Guitar instrument (for guitarrón) and Soprano Guitar (for vihuela). Right click the staff of that instrument, then choose "Staff/Part properties" and click the "Edit string data" button. From there you can change the number of strings, each string's tuning, and the number of frets on the instrument (for tablature). As for playback, if the default sound isn't to your liking, you could search around online for any custom soundfonts containing these instruments, and use those.

Handbook page on editing string data: https://musescore.org/en/handbook/3/tablature#change-tuning
Handbook page on loading custom soundfonts: https://musescore.org/en/handbook/3/soundfonts-and-sfz-files

Once you've set these up in a score, save it to your Templates folder, then it's available for use under the Custom section of the dialog next time you create a score.

Meanwhile, though, I agree these instruments should be added to the default list. We'd just need a definitive definition regarding how they are tuned, how they should be listed in the score when abbreviated, etc.

justamente estoy necesitando de estos instrumentos, espero que algún día los tengamos de forma predeterminada.