Hi sasko
Sorry, no it isn't possible!
Maybe you can use a workaround to solve this problem: Don't use a constant value as the timer set value, use a marker double word MD instead and make this MD remanent (e.g. MB 1 - 4 for MD1).
Now you can write the MD with your OPC client. Note that the value to be written has to be an integer respective to the time range (mins, secs or msecs): If you e.g. want a set time of 2:300 ms (2 secs + 300 ms), the value to be written is 2300.
If it all fails, you have to buy a new one I'm afraid!
Best regards
Bernhard