Fix holes in steel barn roof from the inside?

A. You can try. Your degree of success will depend on the size and type of holes and your technique. If the holes are the result of corrosion, your repair will be only temporary, as the corrosion will continue. I'd suggest you try urethane caulk.

Once corrosion to the point of perforation has set in, the only real answer will be a new roof.

A. The easiest thing to do would be to squirt a small amount of real silicon caulk up thru the hole. A poor fix, but it is cheap-unless you fall.

A. Yes.
Even if you want the patches to be physically on the outside you can repair from the inside.
Cut oval or rectangular holes and feed the larger patches through, short axis to long axis and turn them through 90á.
If you apply a sealant to the mating faces you should be able to make a watertight seal.
Blind fasteners (such as pop rivets for metal) would then be used to hold the patch in place through the overlap.
If you can get the fasteners through whilst the sealant is still wet it will help squeeze the sealant down to make the repair watertight and hopefully create a fillet around the patch to reduce the chance of crevice corrosion.

A. Somewhat old formula is mixture of iron powder (or zinc) and 'water glass' ⇦this on eBay or Amazon [affil links] (thick paste), today you can use 2 parts epoxy cement mixed with iron powder (or zinc powder). Hope it helps and good luck!