Custom mouth visemes?

Is it possible to add a custom viseme so I can manually add it in when I'm editing visemes?

The reason I ask is that I'd really like to have a viseme for K.

I know we're probably supposed to use one viseme for L, K and N  - as they all have similar mouth placement. But IMHO the tongue placement is quite different. For example, If you say the word "color" over and over, you'll feel your tongue going up and down between the K and L consonants. So a character with a special K viseme could say "color" a lot clearer.

There are other custom visemes I might like to add too, including SH, TH, N, T and Y (I could give details of why, if anyone's interested), but K is the main one I feel is missing.