Yes, Designer 7 will directly import Output Designer 5.5 .ifd files. I don't use Designer 7 as Adobe turned a corner with the software that we don't want to take. But a quick test with one of our Output Designer forms indicates that it seems to do a pretty good job of importing everything. It did seem to have a font sizing problem so that some of the text boxes, check boxes, etc. were not imported large enough to show all of the text. You get a big ole white on red plus sign showing that it needs to be expanded.
This was a simple mostly text form. It might have more quirks with more complex forms.
Output Designer is the renamed & reworked update to JetForm Designer. In the process of being updated by Adobe it lost the form filling intelligence which is where it got the "output" label. Therefore, I doubt if it can import any intelligence so you would have to add it all back.
Even if you were to upgrade to version 5.5 you would have to basically do two conversions. First from 5.2 to 5.5, loosing all filling intelligence and I don't know what else since we looked at the older version but when it came time to purchase we got 5.5. If you did that it would import directly but you would still have to rebuild any intelligence.
Unless you have a lot of forms with a lot of fields you are probably doing it the most efficient way.