Clamps compatible with Centameter?

It's a matter of the clamp's technology - are they delivering a voltage proportional to current measured or are they translating current? Both variants exist.
The FLM clamps deliver a well-defined voltage proportional to the current they measure (0 to 5V). This voltage is put into the measuring microcontroller via a voltage divider that corresponds to the impedance of the current clamp (in the FLM case a 1k91 resistor corresponding to the 6k8 clamp impedance) "translating" the 0-5V to 0-1V1 in relation to the Atmel ADC's reference voltage.
So, the answer to your question is that other clamps may work if you are able to provide a "fitting voltage relation" towards the measuring microcontroller, But I doubt that you want to desolder the 1k91 resistors to replace them by an equivalent to your clamps...

Then the first precondition is met; the second is, that the impedance (the clamp's own resistance) correlates with the voltage divider built into the FLM (as it needs to internally bring down the voltage to 0 to 1V1 as the FLM works on 3V, thus does not really like direct 5V input) - if it is around 6800 ohms then it will work - sadly the centameter homepage does not tell the spec of the clamps; but have a look at the clamps themthelves. Maybe there is an indicator who the supplier is to the get actual parameters of the clamps via your preferred search engine... BTW: The FLM schematics are online, so you might help yourself in finding out the physical conditions, it's just school physics ;-)