I have 15k simple products with custom options, is there any way to convert it in to configurable products?
I rembember that there was a module on a client's installation to convert the type of products but it was not really bugproof.
If you mean to convert each product into configurable, you will most probably have to do it through a custom script that will loop products to change the type with SQL queries. But, I think, you'd better create your configurable products and then attach the simple ones to them. It can be done automatically but it is difficult to tell how without a clear idea of the database.
I had to do the same a year ago. I searched a lot as unfortunately, there is no way to do it by default. I'm not good in coding, so were looking over the web for alternative. I have found this video with a tool that converts simple into configurable and custom options dropdowns into attributes and associated products in drop-downs.
I had only 20 products with 2-3 options in each and they offered to do it as one-time service for $349, instead of buying whole application. If you have less goods, you can manually re-create them.