A .deb package is just an archive with smarts. Extract into a temp dir. You'll get a control and data folder.
Unpack each into their own temp dirs.
In the data folder you find all the files that would be installed under the paths specified by the packager. All paths relative to /. You can manually copy them where they go or change as you see fit.
Study the scripts extracted in the control folder. These are maintainer scripts that specify what actions are performed 1) pre-install, 2) post- install, 3) pre-upgrade, and/or 4) post-remove. They usually set ownerships, permissions, or init directories.
Manually do 1 & 2 adjusted for whatever changes you manually make to the recommended install pathing.
Not rocket appliances, just some manual work, but the scripts are your guide.
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