The ISR will trigger at a specific value of the timer. You can set it to 9, for example. Usually this would clear the timer so it would start counting from zero and your ISR will be triggered every 10 counts.
You want to trigger your ISR every 10 but you want this to be 10, 20, 30, 40... because Timer1 is doing something else for you? You can change the value in the ISR - when it gets to 9, change your trigger to 19, then 29, then...
This is poor practice - your ISR will waste a lot of time changing this thing which doesn't need to be changed.
Does this unspecified-other-function need to be called every 100, for example? Then have your ISR keep a count of the times it's been triggered and do the other function after it counts to 10. This is actually how the millis() function works - it is running from a timer which overflows at 255 several times per millisecond and it keeps a count of the number of overflows before it increments the millisecond counter.
Ore use another timer, if the two functions aren't aligned as exact multiples.