yeah you can if you think they are very skinny and not eating... You'd basically have to grind up something like waxies into a paste and feed them via a syringe. If a leo ever got in that shape you'd have to try get some fluid into them as well... preferably with some electrolytes in the water.
As stated though force feeding is always a last resort!
Why the interest? Out of pure interest or do you know a leo getting to this stage?
I do have a leo whos not eating shes only small and i have asked vets advice and they recommended hand feeding baby food through a syringe which i'm doing, but i wondered if you could do it with waxworms, mealworms etc. She is drinking as i have seen her. I have read a few threads where leos have gone off their food and alot have put it down to the weather, all 3 of mine have but was more worried about my little one as the other 2 are quite chunky nice fat tails.
if shes little and needs weight piling on grind up some waxies... I really wouldn't use mealies they are not very good nutritionally. You can even grind up crickets but make sure they are fairly watery, its not very nice but sometimes it gotta be done!
best way i found is in a pestle and mortar....put in like 3 waxworms and about 3 crickets, a small amount of water a little bit of nutrobal and then grind away...is nasty but is works...
if you have no luck and your leo is getting thinner buy some critical care formula or use ensure...both are pretty good. (critical care from the vet, ensure from tesco.)