You could create a Virtual Output (VO). There are a couple of options -
Simple option:
1. Create a separate VO for each valve. For the first valve for 30mins on, the 'Command for ON' would look like this (with a virtual input slider set at 30)
/result.cgi?lu=admin&lp=pw&xi=0:<v>:0:0:0:0:0:0:0
Then create separate VO's for each valve.
A more complex option:
2. Use a State Block to combine characters to create a text string that defines up to 4 valves (limited by the 4 inputs of the State Block).
I can't test this obviously, but use the attached .loxone file as a starting point to experiment.