Yes that's really simple with the NPC Server I think... I think this is the exact script for that:
NPC Code:
if (playerchats&&strequals(#g,police)) {
this.test=0;
if (startswith(jail player,#c)) {
tokenize #c;
if (tokenscount==4) {
with (getplayer(#t(2))) {
this.test=1;
setstring client.timeleftinjail,#v(strtofloat(#t(3)));
setlevel2 jail.graal,32,32;
}
if (this.test==0) setplayerprop #c,The account '#t(2)' is not online;
else setplayerprop #c,The account '#t(2)' has been jailed for #t(3) minutes;
}
else setplayerprop #c,Invalid setjail format, format should be: "jail player accountname time";
}
}
Only thing you'd need then is an NPC in the jail which decreases the timer.