While this is supported by XFA, there isn't any way to do this at design time today in Form Designer.
If the common subform is something you use frequently you can drop it into the Custom tab of the library.. then you'll have it there to drop into any form you need it in, but when you change the common subform, you'll need to manually replace it wherever it's used.
This is something we're hoping to address in the next version of Designer.
If you're feeling adventurous, check out the "proto" support in the XFA specification. This lets you have one piece of a form basically include another piece of the form, but it's not something that Designer supports the creation of.
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SteveX
Adobe Systems