First of all, that's nice to hear!
Secondly, that's a shame..
Here's an idea for you that I just thought of.
Two product pages, A and B.
A is the page you have now,
"Page A" currently gives access to two files.
Glass Masquerade (Win).zip
Glass Masquerade (Mac).zip
Instead, it will give access to four.
Glass Masquerade (Win).zip
Glass Masquerade (Mac).zip
+
Glass Masquerade + DLC (Win).z00 (Part 1)
Glass Masquerade + DLC (Mac).z00 (Part 1)
(Those would be partial encrypted ZIP files, like you would send to avoid e-mail file size limits.)
"Page B", the DLC page, would have two files.
Glass Masquerade + DLC (Win).z01 (Part 2)
Glass Masquerade + DLC (Mac).z01 (Part 2)
This means people who only Purchase from Page A, only gain access to the base game and do not need to download the Part 1 file.
People who purchase from both Page A and Page B, will download Part 1 + Part 2 and extract them (winzip etc automatically sort of stitches them together) which gives them the full game.
I know it sounds a tad convoluted on paper, but in practice all you'd have to do is download two zip files and extract them like normal.
Just an idea I just thought up for you. :)
I know it's not perfect, but it's the best I could come up with.
Failing that, you could simply add "Glass Masquerade + DLC" as it's own price and have it cost the price of the game + DLC
Personally I think that'd be better than people not being able to buy and support the game and get the updates in any way. :)