Welcome to the SonarSource community. . We hope you’ll enjoy it.
About your question, the answer is no, adding custom rules to SonarCloud is not possible, and this is currently not planned in the foreseable future.
If you miss a rule and you believe it can be useful to all developers in the language you’re considering, then I suggest that you submit the rule idea through this forum. If you decide to do so, please provide all the details as you can see on rules already implemented (description, compliant/non compliant code example, rule type, severity, references)