Yes, based on the Tetra standard, you can connect the system to other networks ( analog radio, telephone etc).
I don't think such interconnections are based on the TETRA
standard since many of them already existed long before TETRA (like the aforementioned 4w E&M), and with TETRA being digital the standard would certainly not specify any analog signaling.
You can even participate in the Tetra group call with landline telephone.
Or from a GSM phone, CDMA phone etc., and you can also make or receive private calls to/from virtually any system that can be interconnected somehow, subject to the vendor of the TETRA system and the interfaces available (4w E&M, ISDN etc.).
In the Cassisidian system the feature is called fallback
The Cassidian fallback mode assumes that the TBS is normally connected to their TETRA switch via a transmission link but when said link is interrupted the TBS enters the so-called Fallback Mode.
When the link is up the switch will download parameters like frequencies to the base station, which are then also used in Fallback mode.
In Stand-alone Mode the TBS is configured locally and does not require a connection to a switch first, think single-site system or a mobile TETRA base station for example. I guess that comes closest to a conventional repeater.
Motorola calls it Local Site Trunking btw, not sure if they differentiate further between "fallback" and "stand-alone" mode.