Quite often this kind of requirements can be fulfilled by using wait steps in the workflow. One example could be that you put a "wait for event" step in your workflow template. Then you could trigger this event from your interface (or whatever) with function module SAP_WAPI_CREATE_EVENT.
Maybe I should ask that how do you know that the condition has been fulfilled? Is it a field in a certain table, or? If yes, instead of the "wait for event" step, you could also wait that your condition is fulfilled (a field in your table gets value X). Just take a look into the workflow template and its step types. Wait step is the way to go.
Of course it might be possible that in the SRM/shopping cart workflows, there is already something built-in, which can be then utilized by using some of the BADIs, but if you cannot find anything from the documentation, then choose one of the above-mentioned approaches.
Regards,
Karri