I have a hybrid with flat bars and I am converting it to drops for cyclocross use, it is based on mountain bike chainset and bottom bracket, can I put a road compact and external bottom bracket on it or is the bottom bracket shell incompatable, it will only have a double chainset

what frame?

I did something similar with a Road Rat and ended up using my mountain bike cranks with a sort of faux compact double using Middleburn chainrings, one on the middle ring position, one on the outer – 34/48, I think. Shifts okay with a Sora shifter and a lot cheaper than a new crankset and bottom bracket. I found 32/48 was too big a jump for clean shifting btw.

Didn’t want to mention the make, its a Marin, an old one though, I only use it for the 3 Peaks but they have changed the rules this year and you can’t ride with flats

If the frame is a 68mm BB shell then a road compact chainset will fit. However if the back end of the hybrid is a 135mm spacing and you may find the crank arms are quite close to the chainstays due to the reduced Q factor. It is probably a case of try it and see.

Have you thought about how to sort the brakes, going from flats to drops?

Lots of possible issues, incompatability of:

mtb front mech/sti
mtb front mech/road chainset
brake levers/calipers

BB shell too – tho IIRC Marins are 68 so you’d be OK.

Am using STI levers and frog leg brakes which should be ok

For the shifters have you considered bar end mounted ones?

I think these generally cope with more extreme ratios than STI as you might have a friction/ratchet option, hence why they still appear on some tourers?