I would take expert advice from a boiler person/heating installer, frankly - the steam condensate produced by these boilers contains nitrites.which are produced by the gas burning process. The pipe carrying the steam outdoors (I assume you had yours fitted more than 2 years ago) is prone to freezing in very cold weather, and if the condensate flows over a wall, or onto concrete, or just ends up dripping constantly onto soil, it will eat through mortar and concrete, including the house foundations, something which wasn't realised when these boilers first started being fitted.
Regulations regarding condensate these days include gravity drain pipes straight into the foul water system wherever possible, rather than exuding steam outdoors, to avoid some of these problems. Any pipe outside a house must be well insulated, and clearly, the longer you make that pipe, the more vulnerable it is, and all must be insulated, together with either arrangements to disperse the condensate safely, or using one of the neutralisation systems. If it's a gravity pipe, it needs to be pointing downwards, there's a risk of feedback to the boiler if it isn't.
Since the steam which is exuded contains nitrites and has a ph of bwteen 2 and 4, if you were intending to just extend the pipe into your greenhouse and allow the steam to be expelled within it, that's what'll be going all over your plants. All in all, it doesn't sound like such a good idea, more from compromising your boiler than anything, but as I said, seek the advice of a boiler expert.