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It is possible, but a task that needs investigation and insight in the way both Joomla and XOOPS store their data.
Once you have identified that, it is only a matter of some SQL statements to transfer the information from the Joomla tables to the XOOPS tables.
There are three kinds of data:
1) unique to Joomla
2) unique to XOOPS
3) common to both.
In the common information there is data that is the same and other that needs conversion eg from string or keyed to int.
A conversion script discards the unique elements of the source, copy or converts the common and supplies defaults for the unique data of the destination.
The reverse script (XOOPS > Joomla) will give you information, which fields can be discarded in Joomla and which fields can be transferred with the eventually required (reverse) conversion.