Not sure if I fully understand the problem, but....
If you mean that you want to add extra work on an invoice to regular work that you already do. Then I would just setup another job for the insides for the same address and change/setup a new job type ie. 'Internal Cleaning' or whatever. Then assign an invoice address to both the jobs you are doing.
To do this. When you first enter your job details and you enter the address there is a box ticked that says 'Use this address on invoice', untick this box, then go to the next page, tick the box 'the invoice address is', then click 'create invoice address'. Once you have created the invoice address, you can then select the same invoice address for any other jobs for that customer, so that when you create an invoice in future all jobs for that address will be together. Also, make sure that you also change under 'debt list' details, 'cash collection' to 'invoiced'.(Its one of the last screens in the setup)
I hope this makes sense, and moreso, I hope this actually answers the questions you posed!!
Simon