Altium: Including Text of Evaluated Parameter in Value of Second Parameter
Sure, we do something similar with variants for our boards. In our case, we append the variant name to the end of the document number, but it should work the same with any schematic parameters.

The way we do it is by editing the DocumentNumber parameter in the Parameters tab within the Document Options window, the same way you're inputting the base DocumentNumber. In that field, enter the following:

='ABCD-'+SheetNumber

Now wherever the DocumentNumber parameter is used, it should appear as a string formatted per your specification (i.e., "ABCD-2", "ABCD-3", etc.) However, I'm not sure how it will behave in situations where there is no value for SheetNumber. In our case, if there is no variant defined or the active variant selection is "No Variant", then the string will appear as e.g., "ABCD-[NO VARIANT]". If you want to avoid this, you can always define a custom parameter such as NumberedDocString within that same dialog and set its value equal to the following:

=DocumentNumber+'-'+SheetNumber

That way, DocumentNumber is "ABCD", SheetNumber is your current sheet number, and the value of NumberedDocString wherever it is used should be formatted as you want it.