These may pretty well be written by hand: .btab files are technically the same as .tab files, but written for the bass guitar (hence, containing only the last 4 strings). You may simply rename the .btab file into .tab or .txt and import it into Guitar Pro through invoking File -> Import -> ASCII Tab.
In case GTP should mess up the string order, try opening the .btab files in Notepad and adding two more empty strings above the existing four.