Hey!
Welcome to Dakka. You're more than welcome to stick an extra shoulder pad, arm, head, grenade or whatever to your models- conversions make for some really unique and interesting models. Especially with Orks, you can go pretty wild. I, for example, have each of my Boyz units with different head scuplts, including fez heads, sombrero heads, military berets and russian ww2 hats. As long as you're not changing a drastically from its original specifications go for it
If you're learning how to play at your local shop, I'm sure they'd also be willing to answer any questions relating to painting and/ or modelling (also feel free to ask on here), so ask away.
Best of luck with your Ork army!
Jangus