Calling a different system: We need to read data form our CRM system. Can I configure the "Call" statement parameters so that the RFC calls up the function module in the CRM system instead of in the current system?

Yes, this is possible.
Either you use the "destination=" option in the Call statement; (in this case the client, user and password of the current system are kept);
or you use the "RfcConnect" statement, e.g.

RfcConnect parameters="ASHOST=gx SYSNR=01 CLIENT=100"


RfcConnect parameters=""

In this case you can specify a different client, user name and password, if necessary. After performing the call you need to reset the RFC connection parameters with

RfcConnect parameters=""

since otherwise all following "Call" statements would continue to use the specified connection.