swap -a will add a swap file to the system to use.
swapadd is a shell script which uses the swap command.

You should use the swap command to add a file into the mix.
FYI - you cannot increase the size of the swap file with swap. See the mkfile command. You cannot increase the size of a file already being used as a swap file (unless it is under Veritas control). You would need to create a second file to get the total size you want.