[Rainer101]

While looking for a solution for real table creation in OmniGraffle Pro 5.2b6 or integration an editable table I discovered the following solutions:
The easiest way until now is, to create a table with Nisus Writer Pro. You can use many attributes of Text and table Styles for transferring the table to OmniGraffle. Now you have to copy the table, not only by selecting the table, but marking by dragging with the mouse over the table, then copying the table (with or without contents) and past the table on your canvas in OmniGraffle.
Now you have a table that easily can be modified. Best of all: in OmniGraffle there is a build-in table inspector available, which isn't mentioned at all yet. To view the table inspector select a cell in the pasted table and ctrl/right-click. Choose table in the menu. Now in the table inspector you have the possibility to add rows and columns, to change alignment, cell borders and cell background. Of course you can change the text attributes with the text inspector as well. Especially when you enable "resize to fit" you will get a really self-adjusting table. And you will see, that those tables already have header and footer rows.
I hadn't tried out, whether Link Back is working too. It might work, because Nisus Wirter Pro and OmniGraffle Pro support Link Back.

The other workaround was my first solution and I write down this solution here for those not having Nisus Writer Pro available and you might use any table or spreadsheet therefore.
Choose any shape from a stencil in OmniGraffle (it works with connection lines too), where you can place text in it. Then type any character and ctrl/right-click. Choose table in the menu. The fields in table inspector are only activated, when there is at least one sign in it. Now you can work as described above.
Another solution is, to copy the cells(!) of another table or spreadsheet and copy these cells in the text field in your shape.
So that's at least a solution for real tables and not only a limited construction with the make table command.