newaction {
    trigger = 'mytrigger',
    description = 'My description',
    shortname = "myshortname",
    valid_kinds = premake.action.get("gmake").valid_kinds,
    valid_languages = premake.action.get("gmake").valid_languages,
    valid_tools = premake.action.get("gmake").valid_tools,
    execute = function()
        -- Execute some stuff
        premake.action.call("gmake")
        -- You also can conditionally execute different actions, but remember about valid_* stufff
    end
}

--
-- Rename gmake action into _gmake
--
assert(premake.action.list.gmake, "Your Premake does not have gmake action!")
premake.action.list.gmake, premake.action.list._gmake = nil, premake.action.list.gmake
premake.action.list._gmake.trigger = "_gmake"
 
--
-- Add new gmake action
--
newaction {
    trigger     = "gmake",
    description = premake.action.get("_gmake").description,
    shortname   = premake.action.get("_gmake").shortname,
    valid_kinds = premake.action.get("_gmake").valid_kinds,
    valid_languages = premake.action.get("_gmake").valid_languages,
    valid_tools = premake.action.get("_gmake").valid_tools,
 
    onproject = function(prj)
       -- Your code
    end,
 
    execute = function()
        -- Your code
        premake.action.call("_gmake")
    end
}