You can and should define interPLMN idle neighbours, that makes no problem at all. For example, at country border, there is no reason why you wouldn’t want your MS to have a smooth transition to the next country network.
1/ You should ensure that your IMSI’s are authorized to attach to the neighbour PLMN.
2/ Adding those neighbours mean you will provide roaming traffic to the neighbour PLMN. That should not be for free. (“Roaming agreements”). At least, you should get reciproqual incoming relations.
3/ If the neighbour operator changes its frequency planning, you should be informed.
4/ Define the external interPLMN cell with a negative CELL_RESELECT_OFFSET (CRO=10 and PENALTY_TIME=640 –> CRO=-10dB), high RXLEV_ACCESS_MIN (-80dBm), in order to give it a low priority.