Because of the mix of VB and FB you can't concatenate directly.
You could use SORT to convert (FTOV) your FB to VB, with VLTRIM if you have large numbers of some trailing value.
Then concatenate in a simple COPY.
If desperate for one step, there is ICETOOL.
Even JOINKEYS (concatenate the two VBs on F1, and put some lower value in a temporary extension in the JNF1CNTL, and the FB for F2, and put some higher value. "Match" on the temporary values (there won't be any) and use presence or otherwise to know where to build the output from (make the REFORMAT variable-length, and that's how it works for the "conversion" in this).