I went human fighter with berserker kit and just wondering when am i able to dualclass it to fighter/cleric. And when i do, do i still get to keep my grandmastery mace skill?

only when you reach high enough level to get fighter things back.but yes, you will get it back

You can dual when ever you want. Make sure you dual as soon as you level-up since otherwise you will not get any benefit of the XP lost.
You will have to choose new weapon profeciencies when you dual. You will not get back your old profeciencies untill your beserker levels are re-activated.

Are you sure you can dual? You need 15 str AND 17 wis to be able to dual berserker into a cleric. Then you will be a Cleric only until you surpass your old fighter level, then you will be a true berserker/cleric, but will advance in Priesthood ONLY.

You must dual before some level, the further you advance in berserker, the more powerful you'll be compared to a single class cleric, BUT it will be harder to get berserker abilities back, so your call.

REMEMBER the XP CAP!!! (2950000 XP in SoA)

If you advance in berserker TOO much, then YOU MAY NOT be able to get berserker levels back once you dual. If you have Thorne of Bhaal(HIGHLY RECOMMENDED!) or XP cap disabler (NOT recommende, you will MISS special abilities) then go in berserking as much patient as you have. If you are patient, you will have a truly fantastic char. I did a cleric/mage dual class, he is now 14th level cleric and 24th level mage and he is great! He can cast every high level cleric AND mage spells, but it was rather hard and annoying to build him, BUT eventually he proved deserving that.

Good Luck in Dualclassing! It is FUN! Long live Humans!

After you dual over, don't put any proficiency points into a weapon that your Berserker has points in. If you do, when you regain your berserker class, those points you put in as a cleric do not get added to those from the berserker - they just disappear, so you've basically wasted them.

Thanks for all your help, especially Silverstars reply was VERY helpfull.

Glad if I can be of any help. Just ignore any possible grammar mistakes I can make, since I am from Turkey my English is not perfect.

Hope your dual-classing will prove to be fun and powerful as mine did.