I have a windows hosting plan with Plesk control panel, FTP, email and other popular options already found in common windows hosting plans. I want to know whether I can convert my hosting plan to a L2TP/IPSec or PPTP VPN service or not. Your answers are really appreciated.
I don't know what popular options are.
If you have Windows and you have full admin access to it, then can almost certainly run a VPN off it. You will need a publicly routable IP. Most VPN technologies do not work well through a NAT. If you don't have a real IP from your provider, then you are probably out of luck.
Here is the Microsoft's page for Routing and Remote Access.
If you do not have full access to the box, then you are probably out of luck.
Also, Microsoft has built-in connection limits that are different depending on the edition of Windows you are running. I believe the web edition does not even support RAS.
Unless you actually have RDP access to the server, no. Also, this is almost certainly a violation of your host's TOS.