Hi,
what you are saying is not entirely correct: the concept of units is unknown to a block / terminal, it is an attribute of the 'absolute' figures you move from one terminal to the other.
So if the units associated with the Outflow vector of the digester are kg/m3, then the same units will necessarily be associated with the Inflow vector of any other block that is directly connected to the digester.
If the model associated to the second block uses g/m3 units, you will have to take care of the [b]unit conversion within the model itself[/b].
Now I realise, you may have a problem if the splitter (or combiner) block that receives the outflow of the digester is to receive another input from a block the Ouflow of which uses g/m3, because you cannot write a model containing an IF-THEN statement based on the units of the incoming terminals. First of all because, as I said before, units are unknown; and second because they would be only at design time.
Anyways, I guess the only solution is to have a [b]special model[/b] in which one particular terminal (you may consider to make it visually different) can only be associated with ADM1 type vectors.
Obviously a cleaner solution would be to re-arrange the ADM1 model so that it uses Inflow and Outflow vectors in g/m3 (or g/d) like any other model in the library.