You could script it quite easily.

$pwm_target = 100 # Set for target speed
$pwm_start = 0 # Set for start speed
PWM(D0,$pwm_start)

:loop
IF(GetPWM(D0) < $pwm_target)
  PWM(D0,$pwm_start)
  $pwm_start= $pwm_start + 5 # Change 5 to required amount for speed of increase
ELSE
  Halt() # PWM at required level, stop script.
ENDIF
Sleep(10) # Change sleep to suit speed.
GOTO(loop)

Apologies if the code is a bit messy and not explained too well I'm strapped for time but it should give an idea (hopefully)

Ok, I understand how the code works. Now it's clear. Thanks ;)

Or you can servospeed() command. It applies to PWM as well

for example

ServoSpeed(d0, 4)

PWM(d0, 5)

Sleep(3000) 

PWM(d0, 90)

Even I learn something new from time to time, I did not know ServoSpeed() applied to PWM too, that's much easier :)