When creating a customization of a standard project one can extend an entity for a new field. It is possible to create an Alternative for a screen or section, this way adding the field into an existing screen (or removing some of those that customer does not want to use).
However it is not possible to alter rules or transformations so the new field gets validated. The only possibility is to extend sequential workflows where the rule is being checked. But this is very time consuming and sometimes not enough. E.g. when the rule is used as a Confirmation Rule in a Screen Menu Item.
Alternative versions of XSLT transformations/rules could be introduced in the same way as for screens. This way you could add your own version under the original (but in your package). It would contain a complete copy of the original which you would be able to alter in any way (removing parts, adding parts). The original transformation would not be used in that case.