There is no such option. In that building there would also have to be a directional antenna and would have to point to the author's antenna. There should be no obstacles along the way. WiFi is not a one-way communication (like on TV).
There is such an option You can, for example, have something like that link
If you want to fry the brain or piss off the neighbors around or do the consequences from the telecommunications regulator, go ahead. Because according to the description, it breaks the rules having an output power 50x greater than allowed in this country. It probably has such an amazing quality that kneel the nations!
is it possible to add an external wi fi antenna to the laptop
There are no wifi antennas for the laptop - there are only external (internal) network cards. You can connect the antenna to receive wifi, but you have to disassemble the laptop (to connect to the built-in network cable and then all the mobility of such a computer goes into the head and it is already a desktop computer), the laptop itself must also be able to (the manufacturer of the wifi card did not fix built-in antennas so that they cannot be disconnected without a soldering iron or other equipment).
100 m as the author said it is not so much
In the building? - according to the regulations, the radiated power of the wifi signal in the 2.4 GHz band is ONLY 100 mW (for 5 GHz it is probably 200 mW) - you are sure that a wifi signal with such "power" is able to overcome these 100m in quality sufficient to maintain the connection ? Usually, such free hot spots are up to a stable range of up to 10 m from this place (no one wants to share "free" wifi, since they themselves bear the costs of maintaining it!). Maybe for what you recommended there is no "detective story", but I assure you that a possible visit (and she will be) of the UKE measurement team will knock you out of this idea and be sure that it will last. The fact that the 2.4 and 5 GHz band is not licensed does not mean that everyone in the wifi network with its power and range can do what they want. Please read this and your colleague will understand what you are talking about: