Hi Steve,
Are you attempting to populate a repeating table based on values from specific fields? If so, then dynamically adding rows and setting values for a target row would require you to write code. This is probably one of InfoPath's limitations - please do correct me if I am wrong. Anyway, I would recommend using qRules' Insert and SetValue command for this if you intend to avoid writing code.
Insert - this command allows you to add multiple rows to a target repeating table on a single click of a button, or you could set this command on an action rule to automatically insert rows.
SetValue - this command allows you to set any target field to a value desired; Also works for specific rows from a repeating table.
Let me know if want to check out qRules. I can help you get started.
Regards,