HI Erik,
j use a counter decremented in the ISR of a hardware timer. When the counter reaches zero a flag is set. The task look the flag and know it must execute. Code inside tasks look like :

void Task (void) {

// if timer still running exit
if (timer1 == TIM_RUNNING) return;
// timer expired reload counter and restart
time1 = TIME;
timer1 = TIM_RUNNING;

// now begins the code to be executed...
...
...
}

Massimo.