I have an template in Master DB. For one of its field, of type "Droplink", I want to set source from Core DB (/sitecore/system/Field types/Simple Types).
So that I can get this list for droplink selection in Content Editor. I don't know if its possible to access some thing like this from Master Db.
You can use databasename=SOME_DATABASE_NAME&datasource=SOME_ITEM_PATH
E.g.:
databasename=core&datasource=/sitecore/system/Field types/Simple Types