Has anyone ever found a way of adding a line to an already created Item Fulfilment? eg, Created Fulfilment from SO which has 3 lines, fulfilled 1 line and we want to add the other two lines, is that possible? Or rather has anyone found a solution?

By UI it is possible to do that ?

I was looking to do via SuiteScript

yes but in general if by UI it is not possible ,by Script there is a good chance that it will not be possible..

Understood, wondered if anyone had found a workaround