Can I seed Percona XtraDB cluster from the existing backup?
Yes you can.


For do this first start XtraDB Cluster on current server.
Then create an incremental backup from current full backup with --galera-info switch.(It's very important to use --galera-info)
Install Percona server in second server
Prepare the backup files (both backups.Full and Incremental) on second server
Open xtrabackup_galera_info file in your full backup directory and copy uuid and seqno (for example e0d744f3-c88a-11e6-ad0d-6ed1e3c29e6d:0)
create grastate.dat file in full backup directory and add the following lines

version: 2.1

uuid:    e0d744f3-c88a-11e6-ad0d-6ed1e3c29e6d

seqno:   0

cert_index:
Back up is ready to be copied over to the MySQL data directory.
Stop mysql service
Delete all contents in /var/lib/mysql 
copy-back prepare backup

innobackupex --copy-back /restore/full
chown mysql directory to mysql.mysql
Config galera variables in my.cnf
Start mysql


You can find more information in this topic:

http://severalnines.com/blog/how-avoid-sst-when-adding-new-node-galera-cluster