You cannot. Well, you can but it's risky, risky enough that it comes down to who is your ADC.
As you know by now (hence the question), two or more champions near a minion/champion death results in less experience gain. That is why a common tactic (not necessarly only in LCS) is to leave the ADC alone, in order for him to obtain more experience.
How can I give my AD carry more XP in the laning phase and make sure they still do well instead of staying and soaking it up, or roaming and they start losing?
Well, for one there's Zilean's passive like @Dragonrage mentioned in the comments, but I suspect you want a more definite answer than "play x champion".
What you need to ensure is that your ADC will still be able to farm without you, and that he won't die without you. In other words, you need to ensure that:
- The enemy team isn't freezing the lane (He wouldn't be able to farm safely)
- The enemy team isn't able to towerdive him (He would get killed)
- Your ADC can effectively not push the wave (Would allow the enemy to freeze)
- Your Jungler doesn't plan on going bot soon (If you leave he gets behind)
- The enemy Jungler doesn't plan on going bot soon (3v1 towerdives arent' that risky)
There's also another situation which is jungling, depending on who your botlane is, you can sometimes go into the jungle and take a camp or two to gain some experience if you're zoned in lane. In this case both you and the adc would take a camp and get a small experience boost instead of gaining no exp at all. However this depends heavily on your own jungler's routes and what he's trying to do at that momment.
Finally the most important thing is: You are being useful. There is no point in roaming to let your adc get level advantage if you're getting even further behind. Let's say you roam and your ADC is level 6. Their botlane is level 5 and you're level 4. They are ahead because they will have their ultimates on a 2v1 situation, or on a 2v2 situation before you have your own ultimate ability. As such you should only do those roams if you're being productive (Taking Dragon, Ganking Lanes, Peeling on jungle skirmishes)