user9462: Suppose that I have a wallet with 0.001 bitcoins, from dozens of micro transactions (6000 satoshi in average). That means if I want to spend them, it must be concatenated and the transaction fee would be higher than the actual value?

user3547: You could consider creating a new address with 0.01 BTC on it, then sending the concatenated amount at 0 trx fee to each of the addresses containing the smaller amounts.
So: 

0.01 -> first address 
First address (now including the 0.01) -> second address
second address (now including the 0.01+first address -> third address

... ad nauseum.

9462: <p>Suppose that I have a wallet with 0.001 bitcoins, from dozens of micro transactions (6000 satoshi in average). That means if I want to spend them, it must be concatenated and the transaction fee would be higher than the actual value?</p>
 3547: <p>You could consider creating a new address with 0.01 BTC on it, then sending the concatenated amount at 0 trx fee to each of the addresses containing the smaller amounts.</p>

<p>So: </p>

<ul>
<li>0.01 -> first address </li>
<li>First address (now including the 0.01) -> second address</li>
<li>second address (now including the 0.01+first address -> third address</li>
</ul>

<p>... ad nauseum.</p>
