Converting 8 digit number to a 10 digit number

Try:

=SUM(1*MID(B3,ROW(INDIRECT("1:"&LEN(B3))),1))+B3*100

Entered with Ctrl+Shift+Enter, not just Enter.

Here's one way using a formula in an adjacent cell:

<b>Excel 2002</b><table cellpadding="2.5px" rules="all" style=";background-color: #FFFFFF;border: 1px solid;border-collapse: collapse; border-color: #A6AAB6"><colgroup><col width="25px" style="background-color: #E0E0F0" /><col /><col /></colgroup><thead><tr style=" background-color: #E0E0F0;text-align: center;color: #161120"><th></th><th>A</th><th>B</th></tr></thead><tbody><tr ><td style="color: #161120;text-align: center;">1</td><td style="text-align: right;;">12345678</td><td style="text-align: right;;">1234567836</td></tr><tr ><td style="color: #161120;text-align: center;">2</td><td style="text-align: right;;">54367822</td><td style=";">5436782237</td></tr></tbody></table><p style="width:3.6em;font-weight:bold;margin:0;padding:0.2em 0.6em 0.2em 0.5em;border: 1px solid #A6AAB6;border-top:none;text-align: center;background-color: #E0E0F0;color: #161120">Sheet2</p><br /><br /><table width="85%" cellpadding="2.5px" rules="all" style=";border: 2px solid black;border-collapse:collapse;padding: 0.4em;background-color: #FFFFFF" ><tr><td style="padding:6px" ><b>Worksheet Formulas</b><table cellpadding="2.5px" width="100%" rules="all" style="border: 1px solid;text-align:center;background-color: #FFFFFF;border-collapse: collapse; border-color: #A6AAB6"><thead><tr style=" background-color: #E0E0F0;color: #161120"><th width="10px">Cell</th><th style="text-align:left;padding-left:5px;">Formula</th></tr></thead><tbody><tr><th width="10px" style=" background-color: #E0E0F0;color: #161120">B1</th><td style="text-align:left">=A1&TEXT(<font color="Blue">SUMPRODUCT(<font color="Red">MID(<font color="Green">A1,{1,2,3,4,5,6,7,8},1</font>)+0</font>),"00"</font>)</td></tr></tbody></table></td></tr></table><br />