Manually, by reading options one by one and creating same configuration in RouterOS. There’s no option to import existing config file. Only for certificates and keys, if they are included directly in config (in -----BEGIN/END CERTIFICATE/PRIVATE KEY----- blocks), you can import the file as certificate and system will find this data in there. One more possible problem, OpenVPN in RouterOS doesn’t have some features of original OpenVPN, so if your provider uses something “special”, it may not be possible to use it with RouterOS.