i've design a database for my shop that can handle buy,sell invoice and calculating stock of goods. but i want the database can handle money too like microsoft money does. but also linked to my current shop database. do i have to make a new database or i have to modify my current database? i have no idea to modify my current database so it can handle money in-out too.

Give us a hint as to what database your using.

my current database can handle buy,sell invoice and calculating stock of goods. also expenses (buy invoice) and income (sell invoice).
you know in a store there is another aspect like expenses for salaries,electricity, loan interest , etc (accounting stuff). i want my database can handle this too so i can retreive information about how much money do i have now in bank, my debt to another store(my supplier), or another store's debt to my store(customer). something like that.

What database are you using? Access? Filemaker? What is the name of the application. Knowing this provides insight into answering you initial question.

Yes, you can write your own accounting package and house it's data in your current DB, however, why? There are probably many accounting packages you can purchase that can use your current DB.