In order to dematerialise physical securities one has to fill in a Demat Request Form (DRF) which is available with the DP and submit the same along with physical certificates one wishes to dematerialise. Separate DRF has to be filled for each ISIN number.
Yes. The process is called Rematerialisation. If one wishes to get back your securities in the physical form one has to fill in the Remat Request Form (RRF) and request your DP for Rematerialisation of the balances in your securities account.
Yes. You can dematerialise and hold all such investments in a single demat account.