Hi!
I would approach the needed results by using Product class:
http://kb.x-cart.com/display/XDD/Managing+product+class+attributes
You can create the attribute/option configuration and then assign this set to some product class. Later on, when you create a new t-shirt product, you assign this product class to it and all product attributes/options will automatically added to this new product.
Of course, in order to assign product class to existing products, you will need to use import/export. Our import/export does support product classes.