As it stands, for this level of effort i'd probably only attempt this with CDK and developer support to back you up.
I've seen this question a few and there are some limitations that make it a bit challenging to do this. The first is that we require when requesting machines that they be allocated to some host so we require data collection to obtain all these resources and store them in our resource model database. There's currently no way to turn this off so you'd have to manually add the resources to the database, use our collection to still populate and allocate, or use a test host to creat fake resource for you (I'd realistically recommend the second).
The other is when our provisioning workflows fire they respond back with information specific about the machine object they created. If you were able to get past the above then you'd possibly bypass provisioning to run a custom workflow, but you'd need to make sure your provisioning process in this workflow responded back and updated the model. The provisioning workflow wouldn't be configurable through our standard drop down, so you'd have to stick with our basic workflows and then they'd be skipped.
This is a sample configuration to bypass the provisioning workflow, while you could configure this to run a stub workflow that we ship out of the box, implementing this would probably require CDK and CDK support.
<plugin fullName="DynamicOps.External.RepositoryWorkflows.InvokeRepositoryWorkflow" priority="1">
<MasterWFStateCriteria>BuildingMachine</MasterWFStateCriteria>
<MasterWFTypeFullNameCriteria>*</MasterWFTypeFullNameCriteria>
<ExecuteWhen>PreActivityExecution</ExecuteWhen>
<AssemblyPath>[ExternalWorkflowsDirectory]\DynamicOps.External.RepositoryWorkflows.dll</AssemblyPath>
<AllPropertiesExist>
<Property>Machine.Admin.InterfaceType</Property>
</AllPropertiesExist>
<WorkflowArguments>
<NameValue name="WorkflowName">CustomProvision</NameValue>
<NameValue name="WorkflowTimeout">10:00:00</NameValue>
<NameValue name="FailureState">Disposing</NameValue>
<NameValue name="SuccessState">MachineProvisioned</NameValue>
</WorkflowArguments>
</plugin>