Answer : Yes, of course . As long as the prepaid number is active and valid, you can top it up.
You may not, however, get an SMS alert. Topup credit is not related to the SIM connecting with the network, rather it happens on the account side for a number. You can always contact the call center, to request confirmation is a topup has been received.
You can only use topup codes or scratch codes if your SIM is successfully roaming.