Written by Tom Winterhalder
Yes it is possible by modifying the file teac.front.class.php. In this file just find the function freeDay.
In this function you can modify the definition of holidays, or define new ones.

A holiday definition looks as follow:
// myHoliday occures every 5th May.
$myHoliday=mktime(0,0,0,5,5,$year);

To check if the current date is on a holiday the following check needs to be extended:

if ($currDate==$newYear || ... $currDate==$boxingDay)

By adding || $currDate==$myHoliday.

Written by Tom Winterhalder
Before upgrading the component to a later version I recommend to backup the existing data.
Upgrade
Usually you can just copy all the new files to your webserver and use the upgrade...sql file to modify the existing tables.
You probably need to adapt the sql file prior to use it.

Written by Tom Winterhalder
You can use your component for whatever you want. If it fits your need.
Modifying some text definitions is also possible.
You can find all the definitions in the language files.
i.e. administrator\components\com_teac\language\english.php

To modify some definitions just do the following:
i.e. you want to change the main title "Team availability"
Open the language file in your favourite editor amd find the text sentence you want to change.
DEFINE("_TEAC_LANG_TITLE", "Team availability");
Once found, you can change the text to whatever you want.