Can I use the practice lab site to perform custom labs?
Labs from text books etc?
Practice Lab site ?
You can use the practice-labs site to create your own lab as long as it falls within the parameters of the current physical topology they show you, that is, they will not rewire a lab for your custom needs but you normally can configure the lab how you'd like.
Having said that, some labs have script that pre-configure them for the lab they're created. You may find them to more scripted that a sandbox environment.