Sure, since there is no limit to how many websites you can add to your cPanel, but a shared/budget plan is not the same as a reseller plan.
A reseller plan actually allows you to create separate cpanel accounts, for your clients.
The main risks with adding a client to your account is, if they want to access something such as FTP, you can create an ftp account
for them, but it will include your domain name.
For example: email@example.com
Also, if you add a client site and it is very resource intensive, it can cause your entire account to get suspended. The resource usage tool in cPanel, can help you determine how much CPU you are using.
If you have any further questions, feel free to post them below.