How to work with .install files and hook_enable etc... when developing a module?
Yes You can write in your hook_update_N() like this in your mymodule.install file:

function mymodule_update_7001() {
 db_add_field('table_name', 'field_name', array(
  'description' =&gt; 'The total amount to be paid for the order.',
  'type' =&gt; 'numeric',
  'precision' =&gt; 16,
  'scale' =&gt; 5,
  'not null' =&gt; TRUE,
  'default' =&gt; 0.0,
 ));
}


After writing this you can simply run your update.php file