You could create a new (sub-)folder for that addtl project.

Behind that folder you could create any amount of pages for the addtl project.
The BaseLayer could be used for all pages, or you could create a modifed (or specifique) layer for these addtl pages.

One thing, though, there is only ONE HomePage (index.htm) for the domain/project possible.

It would need an addtl button/link on the main project, pointing to the addtl project.

Actually you could name another page entry point (index.htm) for the addtl project.
Just you could not name it HomePage inside the addtl project, but name it (the file) index.htm in the addl project.

If you (or your clients) would like to access that addtl project directly, it would be required to use the target entry link of that addtl project part. Which is the same as accessing it by a link from the main project.

It could even be just the name of that addtl folder.
pe: 'mainproject' (index.htm), the name of the original base project for the main entry,

and 'mainproject/addtlproject' for the addtl project folder, where the addtl index.htm would be located.
Some misunderstand might arise from SEO, though.

The draw back might be a long updload time for the complete combined project.
But it could be uploaded partly only. Possible also with automatically filtered FileZilla (View...Directory listing filters).
I propose to very careful keep backups.
Probably it might also be required to enlarge the system memory.
SSD recommended.

Hint: partly some information might be carried over by the iframe:
Element: Interactive...External Page/iframe.

Let me try to clarify.

You have pages created with DreamWeaver and others created with Kompozer

You are in the process to create a new site with OpenElement

You want to add links in the OE project to Kompozer and DreamWeaver pages

You want to host pages from the three software under the same domain namme

Is this the situation?