You face the problem how to put a ' character within a 'string_in_ticks'
Either use the ' within "quotes" or escape it with '\''.
Then you have the problem of matching [a-z] and putting a ] character in between. For this you can enclose a part with \( \) and use a \1 reference (and a \2 reference for the next \( \)).
Last but not least, you don't need /g option (repeated substitution attempts per line).

Code:

sed "s/^[A-Z]/['&/; s/\([a-z]\) /\1'] /"

Code:

sed 's/^[A-Z]/['\''&/; s/\([a-z]\) /\1'\''] /'

Colors demonstrate that in the shell the 'string' is split into parts, and the escaped ' are outside the 'strings'.

Code:

sed 's/^[A-Z]/['\''&/; s/\([a-z]\) /\1'\''] /'

--
Just seeing you want the ] at the very end. Then have a third substitution.

Code:

sed 's/^[A-Z]/['\''&/; s/\([a-z]\) /\1'\'' /; s/$/]/'

The good thing with $ is that it does not match a real character so you do not need to put it back.