Hello,
To add new field to the ISPmanager you need to create a xml file with field description. File has to be located in /usr/local/mgr5/etc/xml/ directory and has to have name ispmgr_mod_<custom name>.xml
There is an example, it adds a new field to Templates form.
# cat /usr/local/mgr5/etc/xml/ispmgr_mod_settings.xml
<?xml version="1.0" encoding="UTF-8"?>
<mgrdata>
<metadata name="preset.edit" type="form">
<form>
<page name="main">
<field name="accesstempl">
<select name="accesstempl"/>
</field>
</page>
</form>
</metadata>
<lang name="en">
<messages name="preset.edit">
<msg name="accesstempl">Function access template</msg>
<msg name="hint_accesstempl">This is a hint</msg>
</messages>
</lang>
</mgrdata>
After file creation you need to restart control panel - killall core