Of course it is possible, from `man awk` :

 > command | getline [var]
 >     Run command piping the output either into $0 or var, as above, and RT.

So (Watch the quoting!):

    find . | awk '/txt$/{"wc -l <" $NF " | cut -f1" | getline nl; print nl }'

Please note that the command executed is 

    wc -l <file

To avoid the filename printing of `wc`.

That works for older awk versions also.  
Remember to avoid the printing of a dot `.` with `find .`, that makes the code fail as a dot is a directory and wc can not use that. 

Or either, avoid the use of dot values:

    find . | awk '/txt$/ && $NF!="." {"wc -l <" $NF " | cut -f1" | getline nl; print nl }'