The rules for creating custom magic items are in the Dungeon Master's Guide, table 7-33 and nearby pages.
On page 288, under the Adding New Abilities header, are the rules you need for adding (you guessed it) new abilities to existing magic items.

A Wild armor/shield (Racede of Stone) is subsumed into your new form but the armor/shield bonus stays. It can be used in conjunction with a tunic, so you don't go around naked. My MoMF used to drop a big blanket on the ground before morphing into some giant form, using it as a peplum. Unless you need to morph during battle, I found it to be an ok way to manage nudity.

As an alternative, Beastskin Armor (MIC, p. 7) uses wildshape charges to change shape with you. MoMF gains lots of WS uses, so the solution is viable as long as you don't change shape too often.

Usually bonuses of the same type don't stack

The System Reference Document on Stacking says

In most cases, modifiers to a given check or roll stack (combine for a cumulative effect) if they come from different sources and have different types (or no type at all), but do not stack if they have the same type or come from the same source (such as the same spell cast twice in succession). If the modifiers to a particular roll do not stack, only the best bonus and worst penalty applies. Dodge bonuses and circumstance bonuses however, do stack with one another unless otherwise specified.

An enhancement bonus is a type of bonus. Thus, for example, a fighter wearing gauntlets of ogre power (that grant a +2 enhancement bonus to the fighter's Strength score) who then is the target of the spell bull's strength (that grants a +4 enhancement bonus to the fighter's Strength score) receives only the higher bonus, that provided by the spell bull's strength.

Exceptions exist, of course, but enhancement bonuses aren't one of those exceptions.