This has probably been asked before,
but i do not know the terms to search

Is there a way to connect one of the analog ports on my
Samsung DCS Compact to a VOIP adapter,
the VOIP adapter to internet,
a second VOIP adapter somewhere else on the internet,
and a analog phone to that VOIP adapter.
To let this analog phone call as it was part of my phone system..?

Probably need VPN or Locked IP-address..?

Absolutely.

Look at Multitech

https://www.multitech.com/brands/multivoip

What about this?

https://www.ebay.com/itm/NEW-UNLOCKED-LINKSYS-CISCO-PAP2T-NA-PAP2T-PAP2-VOIP-PHONE-Adapter-SIP-Gateway-/181249415813?hash=item2a334e9a85:g:JLQAAOxy~g5RxomT

would that be one in each "end" or how would it work?
does it need fixed IP address/VPN or does it find each other in some other way?

You can connect a SIP ATA (cisco,linksys, audiocodes ect...) to the CO ports on the Samsung. there are some changes that will need to be made to the CO supervision settings. I cant tell you what those settings are because they will vary from ATA and Service.

I meant to connect SLI - port to a VOIP adapter..
to get a long distance analog connection directly/part of to my in-house phone system.

But i wish also to connect my IP-phone adapter,
wich work with any analog phone.
As a Trunk... is this what you mean is not possible without changes?

The multitech link above will do what you want by the looks of it.

You can't connect 2 ata's together as they are both clients, they don't have a server.

Your other option is to get an asterix box and dialogic cards for the pbx end and an ata at the remote end

Ok, thanks..
Not easy to get hold on such things cheap..
But what about this?

Welltech IAD 161
is a broadband VoIP gateway that supports: 

- SIP or H.323
- 1 FXS port
- 1 WAN port
- 3 LAN ports 10/100 Mbps
- T.38 fax signaling
- Audio codecs : G.711 a-Law/u-Law (mu-Law), G.723.1, G.729A