Yes using Spacewalk, you can manage/create/provision rhel mechines with active subscribtion but managing/provisioning of RHEL systems using Spacewalk will not be supported. If there are issues arising on the client system then those would get support depending on the subscription you have for the client systems.

Spacewalk is the upstream community project for Red Hat Satellite. Spacewalk is supported within an open community via IRC, mailing lists and bug reports, similar to the Fedora community. There is no pay-for support for Spacewalk, nor is it supported by the Red Hat Global Support Services. If your needs require stability and support over access to the latest management technology, you may decide that a Satellite subscription is right for you.

For more details you can refer - http://spacewalk.redhat.com/faq.html#compare

Paresh