you can add more vcpus...at any time..depending upon the esxi and OS license you can do it on live.
else you need to off the vm and give more vcpu..if the esx host is below 50 % 24/7 then that means the host has lot of cpu unused.
so you can add more vcpu to this vm, now almost all the application support multi processing...no issues you can safely give more vcpu also check with the software vendor..any way if the esx host has lot of free resources.. you can give.