This isn't possible unless you are using a Virtual Burner. Two mentioned several times in these forums are Virtual CD and Phantom Burner. These are commercial products - I don't recall ever hearing of a freeware burner.
Unless you are working with Blu-Ray you can share your project to a folder (select Disk and then use the 'Burn to:' drop down to select Folder). You can then use your standard burning software to burn an ISO from that folder - just make sure your burning software is set for media and not data.
Insanity is hereditary, you get it from your children
Also, the free burning utility, ImgBurn, can write an ISO to a DVD, from that VIDEO_TS folder. If I need an ISO and the Project is in PrE, that is my workflow. Now, Adobe's full-featured authoring program, Encore, only bundled with PrPro, can directly write an ISO image. However, it is no longer offered as a stand-alone product.