It's possible, yes, but it just isn't done very often just because boss's is pretty hard to say. I sound like a snake with a stutter when I try to say it.
A contraction is the written version of something done in speech simply because saying the contraction is easier than saying the two words individually. So a contraction that's hard to say is kind of pointless.
I´m just writing some texts for songs. I´d like to aks if it is OK to use full forms as well as contracted forms of "to be" in one and the same text in favor of keeping the harmony of rhytm and text. Somewhere full form fits and somwehere shortened form fits... Would it look/sound strange for native english speakers?