I checked around a bit and found out, that nowadays it is possible to create this configuration using the nmcli tool. In fact, it allows for complete control of NetworkManager. The manual pages for nmcli are very thorough and pretty understandable. See man nmcli and man nm-settings.
To set up the configuration in this question, the easiest way is to edit your current connection profile. Find out the name of the profile from your GUI NetworkManager tool and edit it using the CLI editor (here the name is Ethernet connection):
$ sudo nmcli c edit 'Ethernet connection'
===| nmcli interactive connection editor |===
Editing existing '802-3-ethernet' connection: 'Ethernet connection 1'
Type 'help' or '?' for available commands.
Type 'describe [.]' for detailed property description.
You may edit the following settings: connection, 802-3-ethernet (ethernet), 802-1x, dcb, ipv4, ipv6
nmcli> goto ipv4
You may edit the following properties: method, dns, dns-search, dns-options, dns-priority, addresses, gateway, routes, route-metric, ignore-auto-routes, ignore-auto-dns, dhcp-hostname, dhcp-send-hostname, never-default, may-fail, dad-timeout, dhcp-timeout, dhcp-client-id, dhcp-fqdn
nmcli ipv4> set ipv4.addresses {your_address_here}/{your_network_prefix_here}
Do you also want to set 'ipv4.method' to 'manual'? [yes]: no
nmcli ipv4> save
Connection 'Ethernet connection' (87fa8e41-7fe3-435a-a2f2-29a9c8084d2d) successfully updated.
nmcli ipv4> quit
Bolded portions are your inputs, replace things in curly braces with your settings. Answering no to the question about ipv4.method preserves DHCP configuration. Afterwords you need to disable and re-enable the configuration, which can be done using GUI-tools or nmcli.
If you want to, it is also possible to create a new connection from the command line:
sudo nmcli -p connection add type ethernet ifname {your_interface_name} con-name MyConnection -- ipv4.addresses {your_address}/{your_prefix_length} ipv4.method auto
Again, replace things in curly braces with your settings.
Finally, if you still wish to edit the configuration files, remember to reload the file after editing:
sudo nmcli connection reload
Do note, that configurations created this way are likely to confuse the GUI tools. At best, they won't show you the static addresses.