This question already has an answer here:
Is it possible to create a new default master page in SPD or edit the default one? I dont want to enable SharePoint Publishing Infrastructure as it results in problems with templates. The idea is to create a site collection (1 top site and 3 subs) and then template these out.
According to this link it's possible to direcly go to /_layouts/ChangeSiteMasterPage.aspx and select master page however this works only with SharePoint Publishing Infrastructure enabled.