You can use an extra snippet for configuration.
This is what I usually do. I have a snippet called snippet_name_a, and I have another snippet with snippet_name_a_config
Inside snippet_name_a_config, I have this code
global $snippet_name_a_config;
$snippet_name_a_config = array();
$snippet_name_a_config['param1'] = 'value1';
$snippet_name_a_config['param2'] = 'value2';
While inside the snippet_name_a, I have this code on the beginning, so I can access specific global var from the config
global $snippet_name_a_config;
if(!isset($snippet_name_a_config)) {
// Load all default config parameters needed for the snippet or you can return error, because no config found
}
The way you use the snippet is
[!snippet_name_a_config!]
[[snippet_name_a]]
Or you can define the snippet calling on the snippet itself by using $modx->runSnippet API.
Hope it helps.
Sincerely,