Greetings, I've had a look around the forum and the wiki, but was unable to find any specific info that relates to my situation. I'm working on a crafting recipes mod. I have selected Skyrim.esm as my masterfile and created an active file that I save to. I have created several recipes and they have all tested OK. I have Hearthfire and Dawnguard installed and have successfully tested my mod at Lakeview Manor. I assume that Skyrim.esm only contains items from the vanilla game.

What I would like to do is add items from both Hearthfire and Dawnguard into the recipes that I am making (as well as potentially update the mod when new DLC is released), so that a player with Dawnguard installed could use the Dawnguard item recipes and a player with Hearthfire could use Hearthfire recipes. At the same time, I would like the mod to work for players with just the vanilla game.

Do I need to change my Master file for the relevent recipes? For example: I want to create a recipe using items in Dawnguard, do I restart the Creation Kit, select Dawnguard.esm as the masterfile and my active file, make the crafting recipes and exit? If so, a player with Dawnguard installed is then able to use both the vanilla and Dawnguard recipes, assuming they meet any conditions I may set (e.g. perk requirements)?

Or am I completely off track? Any help would be appreciated.

I could be wrong but I don't think there is any way for plugins to have optional master's. The only way to achieve the compatibility you're talking about would be to have multiple plugins.

So for example: MyModSkyrim.esp, MyModDawnguard.esp and MyModHearthfire.esp? Can these be uploaded as a single package, or would I make 3 seperate mods and upload them seperately?

Edit to clarify: By single package I mean a single mod that a player would subscribe to on the Steam workshop with the 3 seperate .esp files.

Pretty much.

Regarding Steam Workshop I have no idea. I don't upload or download mods there so I can't be of assistance to you.