While it is certainly possible to open the phone and solder wires from the speaker to an external amplifier I would recommend against it.
High-quality speakerphones are a combination of electronic, acoustic, and mechanical engineering working together. One of the key features of good performance is the reduction of coupling from the far-end caller back into the microphone(s). An external amplifier and speaker will drastically alter the phase, direction, and level of the far-end caller as received by the microphone of the speakerphone, resulting in echo, feedback, and generally bad performance.
Spend the bucks for a well-designed conference room phone such as the Polycom Soundstation 5000 or similar.
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