is ist necesarry to do it by "add a custom token"?
Golem is an ERC20 token..as far as i understand you can send it to any Ethereum Address and if you want to send it from your MEW you must click "load token balances"..
I just checked, you do not need to add nothing, there is already token GNT. I think that this adding is just when token is completely new which is not case.
To avoid losing of tokens always send small testing amount to address you want to use, just to confirm that everything is ok.
Your feedback would be appreciated.