Can the case u/s 125 be converted to sec 13 b ???

NO, both the   courts  are differnt in rank  and jurisdiction, se 125  lies  in criminal forum   and se 13b  lies in civil forum , porcedures  are  also  different  

You have the following two options:-

1.      The best legal option for the parties is to withdraw the Sec 125 application by giving joint statement in the court that both the parties has agreed & decided to file for Divorce by Mutual Consent. Then you are requires to file fresh application before District Judge/ Family Court for divorce by MCD. Non party can change his/ her stand after giving statement in the court. If any party changes his/ her mind, still can Refile for the Maintenance u/s 125 CrPC .

2.      Inform the court about the decision of the parties going for MCD and request the court to give a long date in sec Sec 125 Case (Generally court will accepts the request when made jointly by the parties & will give you next date for more than 6 months to enable you to settle the matter through MCD). After taking divorce by MCD (in which either the lum-sum money is given or monthly payment is fixed) file the copy of judgment of Divorce on file of sec 125 case and judge will terminate the case in light of the order of Divorce.

But you have to undergo for the waiting period of 6 months for MCD and it can’t be saved by conversion( which is legally not possible) as you want to.

this is the right way..