NOTE: This has been tested with a Polycom 450 and a VVX300. Please add here if you have tested with other Polycom models but should work for all Polycoms.
Although there is no way to add a speed dial, there is a way to add a soft key using Enhanced Feature Keys (EFK)
<feature>
<feature.enhancedFeatureKeys feature.enhancedFeatureKeys.enabled="1"></feature.enhancedFeatureKeys>
</feature>
<softkey3>
<softkey softkey.2.action="2988$Tinvite$$Cwaitconnect$$Cpause4$*$Tdtmf$" softkey.2.enable="1" softkey.2.label="VM" softkey.2.use.idle="1" />
</softkey3>
Here is another scenario where the speed dial calls a Meet-Me Audio Conference Bridge, inputs the moderator pin, and presses #
<feature>
<feature.enhancedFeatureKeys feature.enhancedFeatureKeys.enabled="1"></feature.enhancedFeatureKeys>
</feature>
<softkey3>
<softkey softkey.2.action="6032$Tinvite$$Cwaitconnect$$Cpause4$8672598#$Tdtmf$" softkey.2.enable="1" softkey.2.label="BridgeMod" softkey.2.use.idle="1" />
</softkey3>
6032 - Internal Conference Bridge Extension
$Tinvite$$Cwaitconnect$ - Waits for the conference bridge to pick up
$Cpause4$ - Waits 4 seconds
8672598# - 6-digit moderator pin and pound
$Tdtmf$ - Use DTMF to dial the pin
softkey.2.label="BridgeMod" = What the softkey will be labeled that uses this code