function MYMODULE_preprocess_user_profile(&$vars){
//    dsm($vars); // prints nice debugging information
    $vars['user_profile']['charity_link'] = array(
    '#markup' => l('Add Charity!', 'my/custom/link' . arg(1)),
    '#weight' => 10
    );
}

<?php
//  unset the original template to set yours.

// Implement hook_theme_registry_alter
function MODULENAME_theme_registry_alter(&$theme_registry) {
  $theme_registry['TEMPLATE_TO_OVERRIDE']['template'] = drupal_get_path('module', 'MODULENAME') . '/TEMPLATE_TO_OVERRIDE';
  $theme_registry['TEMPLATE_TO_OVERRIDE']['theme path'] = drupal_get_path('module', 'MODULENAME');

}

$theme_registry['user_profile'] = array(
  'variables' => array(
    'account' => NULL,
  ),
  'template' => 'modules/user/user-profile',
  'file' => 'modules/user/user.pages.inc',
  'type' => 'module',
  'theme path' => 'modules/user',
  'preprocess functions' => array(
    0 => 'template_preprocess',
    1 => 'template_preprocess_user_profile',
  ),
);

<?php
//  unset the original template to set yours.

// Implement hook_theme_registry_alter
function MODULENAME_theme_registry_alter(&$theme_registry) {
  $theme_registry['TEMPLATE_TO_OVERRIDE']['theme_path']  =>
    drupal_get_path('module', 'MODULENAME'))
}
?>

<?php
// Implement hook_theme_registry_alter
function MODULENAME_theme_registry_alter(&$theme_registry) {
  $theme_registry['TEMPLATE_TO_OVERRIDE']['theme path']  =
    drupal_get_path('module', 'MODULENAME');
}
?>

/**
 * Implements hook_registry_alter().
 */
function lml_node_theme_registry_alter(&$theme_registry) {
  $module_path = drupal_get_path('module', 'my_module');
  // Override the default template with our own.
  if (isset($theme_registry['toolbar'])) {
    $theme_registry['toolbar']['theme path'] = $module_path;
    $theme_registry['toolbar']['path'] = $module_path . '/templates';
    $theme_registry['toolbar']['template'] = 'mytemplate';
  }
}