DHCP Reservation

Solution 1:

According to this definition of scope, if they are not in one of the scopes they would then (implicitly) define a scope - whether that is accepted is up to the server software (which you are not saying) to decide.

Solution 2:

Reserved IP addresses do not need to be within a dynamic IP allocation pool or scope.

This applies to Windows and Linux (ISC) DHCP.

A Linux configuration example where there's a range of .70 through .250, but also static reservations (fixed-addresses) outside of that range at .20, .21 and .22:

subnet 199.3.63.0 netmask 255.255.255.0 {
        authoritative;
        option routers 199.3.63.252;
        option tftp-server-name "199.3.63.3";
        next-server 199.3.63.3;
        filename "/dsl/pxelinux.0";
        range 199.3.63.70 199.3.63.250;
}

host chasea {
        hardware ethernet 00:80:d4:05:20:d6;
        fixed-address 199.3.63.20;
}

host chaseb {
        hardware ethernet 00:80:d4:05:21:0c;
        fixed-address 199.3.63.21;
}

host chasec {
        hardware ethernet 00:80:d4:05:21:0a;
        fixed-address 199.3.63.22;
}