You'll have to do it in two steps. Use bin2dec or base2dec to convert from your base to decimal, and then use dec2base or dec2bin to convert to the other base. sb2 = '100001000111001';
sb7 = dec2base(base2dec(sb2, 2), 7)
Note that I've ignored the extremely odd grouping that you've used in your example. If you want to use that grouping, you will have to reshape your string into rows of 3 column and pad the last row with 0.
Your grouping does not make sense to me though since the binary numbers 100 001 000 11 100 001, 100 001 000 11 100 01 and 100 001 000 11 100 1 would all have the same representation (and also note that you've only got two bits in the 11 group).