Transfer data from standard mysql server to Percona XtraDB Cluster
If you have disk files, then the best would be to install 5.5 somewhere in order to get at the data.  5.7 is probably too advanced to be able to read 5.5 files without some hiccups.

If you have a data dump, then it will probably load into 5.7 with minimal hiccups.

A PXC node can be a Slave to your Master.  If we can get that set up, and if there are not conflicts with table names, etc, then we can feed the data into the cluster this way...  After setting up a Master with no data, "load" the data from a backup of your 5.5 data.  Probably (but I am not sure) a 5.5 Master should be able to replicate to a 5.7 Slave on a PXC cluster.

Or, it should be possible to simply load the backup directly onto any PXC node.  (Or the designated Master node, if you configured it that way.)

Do you have GTIDs enabled?  Galera type?  Or Oracle type?  (If enabled this may be another hiccup point.)

Sorry to be vague, but I hope I have given you some tips on where to look next.