Tomatoes in Greenhouse borders

From my own experience yes you can dig in manure or garden compost now or later depending how mature the manure is. I grow toms in my beds in the greenhouses and polytunnels and would not go back to pots or grow bags for just the reason you say - watering. It is still important not to be erratic with watering but an occasional miss is never as important with beds as it is with bags and pots.
Best of luck
R