I've done it many times.
Are you planning to mig it or tig . If you have eccess to a tig that would be my choice,
with a tig you can use either 304-308-309-316 s/s filler rod.
If you are concerned about strenghts, 309 filler rod, or mig wire.
For most applications 304 filler rod, or mig wire,
Or if you plan on arc welding it, the same s/s welding rods numbers would be efficient for what you are doing.
The important thing is preparation. A weld is only as good as the preparation.
The material should be beveled or v'd out if you will, flat bar to pipe might not need to much bevel.
If you need to sand and polish it after, you might need to bevel the flat bar for proper penetration, cause you sand away part of the weld.