I'm working on a transition between two songs right now. The issue is that the end of the first song is in B major, whereas the beginning of the second song is in B minor. I was thinking about adding a progression to the B major progression to modulate it to B minor.

Anyone have some suggestions on this?
How goes the progressions in each songs? 'Cause if they are the same intervalls you could try to just chance it from major to minor as Satriani did in Always with me, Always with you.
just end the B major progression with good old F# and then just start the b minor one and everyone will be like oooooooh dammmnn he's got those 5ths downnnn
major b c# d# e f# g# a#

minor b c# d e f# g a

I'm presuming it's natural minor. A common method is to use a common chord, but in this case there isn't one. I'm also presuming that the progressions are diatonic

However, it can sound good simply to flatten the 3rd in the b major and go straight into the next song.

It is very common practice to borrow chords from the parallel major or minor. So harmonise both scales and play with those chords to find something that sounds good to you.

I can't hear the song, so can only suggest things to try. They may not work.
So you basically want a transitional series of chords to lead you to the parallel minor? But as others have indicated, even short of a specific part for a transition, the V chord (F# or F#7) will lead back to I and you can just go straight into the parallel minor that way if you wanted. As far as trying to come up with ideas for a transition, I'd need more info to work with.
Cool, thanks for the ideas guys, I will toy around with that a bit and see what I come up with.