Unfortunately they dont '(most of them). However, I found a workaround for the Allnet Powermeter 3690. I wrote a litte Powershell script which does some extraction of the ampere-values of the mainpage of thouse devices, and retuns them to PRTG in three different channels. The respective Powervaluecalculation can then be done by a formula-sensor. Works prettyy well for me, but rewuires Version 2.x Firmware on the devices.

# Test and Demo PowerShell Custom Sensor for PRTG
# Just a dirty hack using Powershell
# Requires: 
# + PowerShell v. 2.0 on the local Probe
# 
# Required Parameters:
# + %device %linuxuser %linuxpassword (take care on the sequence)
# In case user/pass is required use the ssh user/pass, becaus the api
# does not provide webuser/webpass as parameters or give user and pass directly
# as commandline parameters 
#
# Hardcoded (covers Firmwarerev. 2.x, setting for FW 1.x may differ):
# + deviceurl : /std_display.php?s=1
#
# Written and Cpoyright by: Andreas Hümmer <andreas.huemmer@elaxy.com> 
# Elaxy BSS GmbH & Co KG 
#
# Version
#  20.11.2011   V 0.1  initial release
#

$DEVICE=$args[0]
$USER=$args[1]     # not used in this version
$PWD=$args[2]      # not used in this version
##echo $DEVICE
$URL="http://"+$DEVICE+"/std_display.php?s=1"
##$URL = "http://sys-cob-pom-001.elaxy.org/std_display.php?s=1"
$webclient = new-object System.Net.WebClient
$webpage = $webclient.DownloadString($URL)

$webpage = $webpage.replace("PowerMeter.findAndCreateClocks(","#####") 
$webpage = $webpage.replace(");","#####") -split("#####") 

##$webpage[1]

$amps = $webpage[1] -split(",")

##$amps
$amps[0]= $amps[0].replace("'","")
$amps[1]= $amps[1].replace("'","")
$amps[2]= $amps[2].replace("'","")
$amps[0]=$amps[0].replace(".",",")
$amps[1]=$amps[1].replace(".",",")
$amps[2]=$amps[2].replace(".",",")

$amps[0]= [System.Convert]::ToDecimal($amps[0])
$amps[1]= [System.Convert]::ToDecimal($amps[1])
$amps[2]= [System.Convert]::ToDecimal($amps[2])

 "<prtg>"
    "<result>"
        "<channel>"
            "Strom Phase 1"
        "</channel>"
        "<value>"
            echo $amps[0]
        "</value>"
        "<FLOAT>1</FLOAT>"
        "<CustomUnit>A</CustomUnit>"
    "</result>"
    "<result>"
        "<channel>"
            "Strom Phase 2"
        "</channel>"
        "<value>"
            echo $amps[1]
        "</value>"
        "<FLOAT>1</FLOAT>"
        "<CustomUnit>A</CustomUnit>"
    "</result>"
    "<result>"
        "<channel>"
            "Strom Phase 3"
        "</channel>"
        "<value>"
            echo $amps[2]
        "</value>"
        "<FLOAT>1</FLOAT>"
        "<CustomUnit>A</CustomUnit>"
    "</result>"
    
 "</prtg>"

For the All4000 I found an older version of a custom sensor, but unfotunately it seems to habe trouble with the most recent firmware. Either way, as the ALL3000 did, die ALL4000 (and I guess all devices in that family) have a xml-interface [devicename]/xml, which can easily be used by the xml.sensor of PRTG.