I believe it should be possible to add existing ticket charges to an estimate. An example of why you would want to do this is when you perform some work for a client (eg. investigate performance problems on a computer system onsite) and then you need to prepare a quote for the repair of the problem in the workshop (eg. replace bad drive), the client often wants to know how much the entire job will cost if they ask for a quote. If you've already added ticket charges on the fly, then you need to add them again to the estimate, and then remove them from ticket charges later if they accept the estimate, which is a little messy.