Hi again and thanks a lot to all of you for your answers, 
@nevj :
I thought that, too, but theoretically it can be done (see below).
Well, I compiled it from source (see steps in my post #1 ). Now wine-8.0 resides in a dedicated folder. The problem is how to integrate it into playonlinux… 
You can actually make playonlinux use a variety of wine versions. It just depends on what is on offer.
The tab “tools” —> “manage wine versions” triggers a new window which lists all available wine versions ready for installation within playonlinux…
… but for 64 bit versions it stops at 6.17-staging.
@artytux :
thanks for the link. I´ll look into it.
@daniel.m.tripp :
Seems you had better luck with solving your “problem”. 
Great.
@all:
I also posted my query in the playonlinux forum (here and here)
It seems it can be done:
Now let’s give a practical example for a better understanding. At the time of typing this there is Wine 7.0~RC2 available but in POL there’s nothing above 6.18 in the x86 section and 6.17 in the x64 section. I’ve been following the devel branch lately so I installed this 7.0~RC2 devel version.
- create a folder named i.e. Wine7.0 somewhere in ~/Downloads or wherever you see fit
- download there all x64 devel_7.0~rc2 files from the Wine repository here (except for those containing dbg or winehq in their names)
- download there all x86 devel_7.0~rc2 files from the Wine repository here (except as above)
- create two folders: ~/.PlayOnLinux/wine/linux-amd64/7.0 and ~/.PlayOnLinux/wine/linux-x86/7.0
- open each of the downloaded packages one at a time (through Archive Manager, not GDebi), select all available folders within ./opt/wine-devel/ and drag them to their corresponding folders created earlier - 32bit to 32bit folder, 64bit to 64bit folder
If everything went well there will be four subfolders in each 7.0 folder: bin, include, lib64, share in the amd64/7.0 folder and bin, include, lib, share in the x86/7.0 folder - each with their own files.
Now opening Tools > Manage Wine versions from POL’s main menu you should see version 7.0 installed for both 32bit and 64bit Wine. Proceed with creating your prefix(es).
plus:
When manually installing a 64bit Wine version in PlayOnLinux, make sure that you copy these files ‘wine’ and ‘wine-preloader’ from the ~/.PlayOnLinux/wine/linux-x86/{wine_ver}/bin folder into the ~/.PlayOnLinux/wine/linux-amd64/{wine_ver}/bin folder
and copy the ~/.PlayOnLinux/wine/linux-x86/{wine_ver}/lib subfolder into ~/.PlayOnLinux/wine/linux-amd64/{wine_ver}/,
otherwise you will get problems. In the 64bit Wine versions which can be downloaded by PlayOnLinux itself, these aforementioned files and lib subfolder have already been included.
I tried that of course but couldn´t get the same results:
quoting myself as an answer:
I followed your instructions very precisely for installing devel_7.0~rc2 but there is a point which differs from your instructions.
You wrote
> If everything went well there will be four subfolders in each 7.0 folder: bin, include, lib64, share in the amd64/7.0 folder
Well, there isn´t, at least not in my case. There´s just share and bin but no include and no lib64.
The same with the x86/7.0 folder: just share and bin.
Any guess why that ´s the case?
> When manually installing a 64bit Wine version in PlayOnLinux, make sure that you copy these files ‘wine’ and ‘wine-preloader’ from the ~/.PlayOnLinux/wine/linux-x86/{wine_ver}/bin folder into the ~/.PlayOnLinux/wine/linux-amd64/{wine_ver}/bin folder
These files are not there either.
That´s where I got stuck… 
Still: thanks a lot to all of you.
Many greetings from Rosika 