Hi @lucky10101 you can transfer your Shares from Zerodha to Dhan. We have written a detailed not on how to do that here.
Now for margin requirement, there is a rule that a client must give atleast 50% of Cash or Cash Equivalents (G-Secs, SGBs, and Liquid ETFs). So if your margin requirement for a trade is Rs. 5,00,000, then you can give upto 50% i.e Rs. 2,50,000 as Non-Cash (which includes your stocks) and remaining Rs. 2,50,000 needs to be funded by Cash or Cash Equivalents.
Note that All stocks have a haircut deduction and after that they are valued for pledge purpose. For example, if your Titan shares are worth Rs. 2,00,000 and the haircut on the same is 12.84%. So you will be able to only get Rs. 1,74,320 of the value. The details of haircut can be found here.