Such is a common practice among CBG players. For example, a typical GDg tuning is often achieved by using ADg strings and tuning the A down one note to G.
Many strings can be tuned to within 1 or (pushing it) 2 notes beyond key. This may shorten the life of the string, or you may notice no difference (except tuning an A down to G makes it "softer" and easier to press).
DAd tuning is done using the DGe strings, with the G tuned up one note and the e tuned down one note. Alternately the EAD strings can be used, tuning the E down to D. Note that this method is "all bass", cutting out the treble side of the guitar.
So yes, strings can be tuned off a bit... especially quality strings such as D'Addario (not to promote any one brand, but in conversations with other builders that brand seems to be highly favored). That's not the only good brand; different players will favor different strings, for various reasons.
You can find out more about tuning and playing here: http://Wishbringer.spruz.com