Until Romania is formally entered into the Schengen area they should automatically stamp your passport as you enter or leave, so you can indeed request it if they don’t.

Hi. So I can ask on arrival at Arad for someone to Stamp it?

You don’t have to ask, they so or so stamp 3rd country citizens passports

I’m not sure they do when travelling overland ? But I hope they do. Cheers Andrew 

You will need to ask the passport inspector as he inspects your passport if he doesn’t stamp it. They are the only people who can stamp a passport - normally as they check your passport electronically. There may also be a Hungarian Inspector.