Would it be feasible to have Space Marines walk outside on the hull of a ship? Or say the hull was penetrated during a ship to ship engagement. Would the marines suit be sealed enough to keep them from dying?

Short answer: If it wasn't damaged, yes, if it was, maybe.

Longer: Armour is enviromentally sealed - they'd be fine if it was intact. If it was damaged, then Astartes possess the Mucranoid and the Multi-Lung, which would protect them from all vacuum damage save that from depressurisation.

AlphariusOmegon7 is right; power armour (including 'civilian' varients) is equal or better than a purpose-built voidsuit in the vacuum of space, barring unusual environmental conditions like radiation storms. And by 'damaged' he means 'taken enough damage from one hit to overcome Armour value'- then the suit is no longer consifered 'sealed'. That's why all Marines carry that can of sealant as standard equipment.

…and don't forget: they're Space Marines! Zero-g outer space combat is their specialty! They hurl themselves across the void, grab enemy ships, cut their way inside, and win! Can't shoot down a boarding craft if there is no boarding craft…..

If you follow a description make in the novel "Know No Fear" there is a really light kind of atmosphere between hulk and energetic fields of a voidcraft.

In the Novel Guilliman who is of course a primarch survive there for a certain time.

So it wil not be impossible for SM to do the same. It will hurt a lot, take fatigue then fell and die frozen at the time but possible to surviove on a hulk even with a breach armor.

It is your game so do it if you feel it, but could be a nice race against time to find a way in in short order.

As long as the armour isn't breached, a Space Marine can survive in the Void for a long time, due to the Sus-an Membrane and suspended animation, as seen in the third Gray Knights novel.