No, there are no migration tools available for backup data that was taken with other 3rd party backup applications, however should you wish, one alternative would be to restore old backups of your VMs from your old application to a sandbox environment, and backup those newly restored 'old' VMs using Altaro VM Backup.

For example if you have 6 months worth of backups from your old backup app, then you may wish to restore the end-of-month backup versions (x6 versions) and back those up for safe keeping with Altaro.

Of course in this case the dates/time stamps of those backup versions is going to be lost, so these should be noted down for future use.

Pro Tip:  Once you have restored the old VM, rename the VM and include the original backup date in the name of VM, this should make recovering that version easier in the future.