I have a guitar chord progression: F# sus2, C# minor, B minor, E sus2. Over this I have a nice melody played within the c# pentatonic (starting at fret9). I really like what I have so far and wish to develop a chorus, however I'm stumped as to the additional chords I can use, I guessed I was in the key of c# but the chords in this key don't seem right. Any advice would be appreciated.
The Bm chord has the note D as its third, so you aren't in the key of C# minor, but in the key of F# minor. So your progression is actually:
|| F#sus2 | C#m | Bm | Esus2 ||
The reason why the C# minor pentatonic scale works is because it is (also) a subset of the F# (natural) minor scale, which is the basic scale fitting all 4 chords of your progression.
Matt's answered the first part, here's the second. In the key of F#m, relative to A maj., the easily usable chords are : A, Bm, C#m, D, E, F#m, G#o. Another set is available to add interest : the parallel minor set of Am, Bo, C, Dm, Em, F, G.
Using sus chords will give another dimension, by taking away the 'gender' of the chord - maj/min. But don't use one as your last chord...
As you already do have a minor chord on the V (C#m) you could, at the end of your verse, turn it into a C#m7 and add a little ii7 - V7 - Imaj7 progression to give your chorus some new kick to shine.
-> C#m7 F#7 Bmaj7
and use the chords of your new tonic B (or even Bm)...
or add a little bridge between verse and chorus that executes that modulation... ;-)
Later on you can easily turn back to your original key with a e.g. C#7 and continue with your open sounding sus2 progression...