Sorry to here that it is a limitation. Hopefully that feature will make it into the next version.
With that, I see that the .metrop file is simply an XML file. One could easily add another ICON node, but what is the DATA and DATAFONT attributes format? What do all these numbers represent? Can I produce an image from another program and export that to a specific format and get this data? If so, can I use to produce this so I can create my own images?
<Icon Name="transport" HasCharacterMap="false" FontName="transport" IsDuplicate="false" ASCIIValue="e7cb" AlphaNumericValue="" FiValue="transport" GroupName="Shopping" IsDirty="false" ExportCommand="MetroGraphicsPackage.IconCommand" Data="M25,16C23.896973,16 23,16.896973 23,18 23,19.103027 23.896973,20 25,20 26.103027,20 27,19.103027 27,18 27,16.896973 26.103027,16 25,16z M9,16C7.8970003,16 7,16.897 7,18 7,19.103 7.8970003,20 9,20 10.103,20 11,19.103 11,18 11,16.897 10.103,16 9,16z M10,10L15,10 15,12 10,12z M21,7L26,7 26,9 21,9z M7,7L15,7 15,9 7,9z M19,4L19,16 21.537939,16 21.579811,15.927251C22.282034,14.772614 23.552277,14 25,14 26.447723,14 27.717966,14.772614 28.420189,15.927251L28.462061,16 30,16 30,10.15 25.037,4z M4,4L15,4 15,6 4,6z M2,2L2,16 5.5379486,16 5.5798235,15.927248C6.2820587,14.772611 7.5523119,14 9,14 10.447688,14 11.717941,14.772611 12.420177,15.927248L12.462051,16 17,16 17,2z M0,0L19,0 19,2 25.994,2 32,9.4429998 32,18 29,18C29,20.205994 27.206055,22 25,22 22.793945,22 21,20.205994 21,18L19,18 13,18C13,20.206 11.206,22 9,22 6.7939997,22 5,20.206 5,18L0,18z" DataFont="M846.3,283.8C846.3,249.3 818.3,221.3 783.8,221.3 749.3,221.3 721.3,249.3 721.3,283.8 721.3,318.3 749.3,346.3 783.8,346.3 818.3,346.3 846.3,318.2 846.3,283.8z M346.3,283.8C346.3,249.3 318.3,221.3 283.8,221.3 249.3,221.3 221.3,249.3 221.3,283.8 221.3,318.3 249.3,346.3 283.8,346.3 318.3,346.3 346.3,318.2 346.3,283.8z M315,471.3L471.3,471.3 471.3,533.8 315,533.8 315,471.3z M658.8,565L815,565 815,627.5 658.8,627.5 658.8,565z M221.3,565L471.3,565 471.3,627.5 221.3,627.5 221.3,565z M784.9,721.3L940,529.1 940,346.3 891.9,346.3 890.6,348.6C868.7,384.7 829,408.8 783.7,408.8 738.4,408.8 698.8,384.7 676.8,348.6L675.5,346.3 596.2,346.3 596.2,721.3 784.9,721.3z M127.5,658.8L471.3,658.8 471.3,721.3 127.5,721.3 127.5,658.8z M533.8,783.8L533.8,346.3 391.9,346.3 390.6,348.6C368.7,384.7 329,408.8 283.7,408.8 238.4,408.8 198.8,384.7 176.8,348.6L175.5,346.3 65,346.3 65,783.8 533.8,783.8z M2.5,283.8L158.8,283.8C158.8,214.9 214.9,158.8 283.8,158.8 352.7,158.8 408.8,214.9 408.8,283.8L596.3,283.8 658.8,283.8C658.8,214.9 714.9,158.8 783.8,158.8 852.7,158.8 908.8,214.9 908.8,283.8L1002.6,283.8 1002.6,551.2 814.8,783.8 596.3,783.8 596.3,846.3 2.5,846.3 2.5,283.8z">
<Settings FontFamily="Webdings" Character=">" IconBrush="#FFFFFFFF" FlipCommand="MetroGraphicsPackage.IconCommand" SldierValueChanged="MetroGraphicsPackage.IconCommand" FlipX="1" FlipY="1" IsLinked="false" ContentHeight="26" ContentWidth="26" SizeIndex="4" MainWidth="48" Angle="0" MainHeight="48" CustomSize="48" MaximumPadding="15" BackgroundBrush="#FF000000" IconShape="Square" IsBackgroundVisible="true" IsRect="false" CustomWidth="48" CustomHeight="48" Padding="11" />
</Icon>