You can always try adding this information, formatted for your network, where it would match at the bottom of your openvpn.conf.mas file in /usr/share/ebox/stubs/openvpn/:
<%def advertisedNets>
<%args>
@nets
</%args>
% foreach my $net (@nets) {
% my ($net, $netmask) = @{ $net };
push "route <% $net %> <% $netmask %>"
push "dhcp-option GATEWAY 1.1.1.1"
push "dhcp-option DNS 1.1.1.1"
push "dhcp-option WINS 1.1.1.1"
push "dhcp-option NBT 2"
push "dhcp-option DOMAIN octen"
%}
</%def>
Then restart OpenVPN in eBox and then your clients. I don't actually know if the GATEWAY line is correct.